In general, the LCD used for desktop and notebook computers, personal digital assistants (PDAs) and webpads adopts a driver unit to output high voltage and a piezoelectric ceramic transformer to light up a cold cathode fluorescent lamp (CCFL). The prior-art driver unit is described as follows:
The common driver for driving to light up the CCFL as shown in FIG. 1 comprises: a power supply unit, a pulse width modulation (PWM) controller, a driver unit (MOSFET), a piezoelectric ceramic transformer and a loaded cold cathode fluorescent lamp. If the input voltage is turned on, the driver unit will immediately drive the piezoelectric ceramic transformer to operate and the cold cathode fluorescent lamp to be lit by a negative/positive transformation effect, and a pulse width modulation (PWM) controller outputs a resonant frequency by a tube current of the electric current feedback detecting lamp, and the average current of the CCFL tube can be controlled by the driver unit and the transformer. Therefore, the light produced can be projected on the screen of a backlit display.
The magnitude of the output power of the piezoelectric ceramic transformer determines the lit-up length according to the magnitude of the outputted voltage, the equivalence of being loaded in the same cold cathode fluorescent lamp and the output power. Since the display (particularly the LCD TV) tends to be specified and designed in a large size, the CCFL must adopt a long fluorescent tube if the short fluorescent tubes are not arranged alternatively. However, a long fluorescent tube requires more current and higher power. The inventor of the present invention disclosed a “Multiple sets of load driver circuits being applied for piezoelectric transform circuit of long fluorescent tube” in the R.O.C. Patent Publication No. 557,073, which uses two piezoelectric ceramic transformers being connected in parallel and having opposite polarities of the power input ends to receive a voltage drive signal of the driver unit with same phase and output the voltage of an opposite phase to the CCFL by a push-pull method, so that the output current can be even and features a larger power output and better impedance.
Although such patent can solve the power issue of the long fluorescent lamp, the quantity of lamps has to be taken into consideration besides the length, since the quantity of lamps increases as the size increases. In that patent, each lamp must have a set of piezoelectric ceramic transformers, not only increasing the cost, but also creating a problem of increasing the operating temperature and the size of the circuit board to manufacturers.